The long term objective of this proposal is to elucidate the neuroendocrine mechanisms that regulate testicular function in the rhesus monkey, a representative primate. Many of the control systems that govern reproductive processes in mammals, including the neuroendocrine axis regulating testicular function, exhibit species differences, and it is therefore to be anticipated that studies of reproductive physiology in the monkey may have direct relevance to our understanding of the control of human fertility and to the treatment of reproductive diseases in men. The focus of the present proposal is to examine the dynamic operation of the FSH-inhibin feedback loop and to evaluate the significance of this servo- system in regulating testicular function. Five Specific Aims will be addressed: l) To begin to examine the molecular biology of the negative feedback action of testicular inhibin on FSH secretion in the monkey; 2) To investigate the dynamics of inhibin induced suppression of FSH secretion; 3) To fully characterize the circulating forms of inhibin in the male rhesus monkey and to determine whether testicular secretion of an alpha-inhibin precursor peptide is a hallmark of exaggerated FSH stimulation; 4) To determine the relative role of LH and FSH in maintaining inhibin secretion by the testes; 5) To examine the notion that FSH plays an important regulatory role in governing testosterone secretion by the monkey testis. The experimental model to be employed is the juvenile male rhesus monkey, in which an adult pattern of endocrine activity in the pituitary-testicular axis is imposed by an invariant intermittent iv infusion of GnRH. For this purpose, prepubertal animals will be implanted with indwelling venous catheters, fitted with nylon jackets, and maintained in remote sampling cages that permit continuous access to the venous circulation without tranquilization and without restraint. Using this so called hypophysiotropic clamp preparation, the pituitary-testicular axis will be perturbed, using surgical interventions, passive immunoneutralization, and recombinant hormonal probes. Secretion of the pituitary gonadotropins and testicular hormones (inhibin and testosterone) will be routinely monitored using RIA. Circulating forms of immunoactive inhibin will be characterized using gel filtration and HPLC. Steady state mRNA levels for the pituitary gonadotropin subunits (LHbeta, FSHbeta and alpha), and the pituitary inhibin subunits (alpha, betaA and betaB) will be assessed by Northern blotting with monkey and human cDNA probes, respectively. Additionally, bioassays for LH, FSH and inhibin are available as needed. Surgical procedures will be conducted using standard aseptic technique with appropriate anesthesia and postsurgical analgesics.